[DebugInfo] printCompactDWARFExpr: don't assert on stack size

[scott.linder]

Summary

Gracefully handle non-1 stack sizes in printCompactDWARFExpr rather than
assert. Add support for DW_OP_nop and test the zero-sized stack case.

This is intended to be nearly NFC.

[Orlando]

Accepting and gracefully handling invalid expressions in printing code SGTM.

[dblaikie]

Not the best error handling (like might be better returning Error from this function?) but probably OK.

[scott.linder]

In D147269#4241315, @dblaikie wrote:

Not the best error handling (like might be better returning Error from this function?) but probably OK.

I agree, but to avoid shaving too many yaks I adopted the existing mechanism for error handling in the function, which is to write an error message to OS and return false.

llvm/lib/DebugInfo/DWARF/DWARFExpression.cpp

while (I != E) {
     }
     case dwarf::DW_OP_stack_value: {
       // The top stack entry should be treated as the actual value of tne
       // variable, rather than the address of the variable in memory.
       assert(!Stack.empty());
       Stack.back().Kind = PrintedExpr::Value;
       break;
     }
+    case dwarf::DW_OP_nop: {
+      break;
+    }
     default:
       if (Opcode >= dwarf::DW_OP_reg0 && Opcode <= dwarf::DW_OP_reg31) {
         // DW_OP_reg<N>: A register, with the register num implied by the
         // opcode. Printed as the plain register name.
         uint64_t DwarfRegNum = Opcode - dwarf::DW_OP_reg0;
         auto RegName = GetNameForDWARFReg(DwarfRegNum, false);
         if (RegName.empty())
           return false;
default:
            << (int)Opcode << ")>";
         return false;
       }
       break;
     }
     ++I;
   }
 
-  assert(Stack.size() == 1 && "expected one value on stack");
+  if (Stack.size() != 1) {
+    OS << "<stack of size " << Stack.size() << ", expected 1>";
+    return false;
+  }
 
   if (Stack.front().Kind == PrintedExpr::Address)
     OS << "[" << Stack.front().String << "]";
   else
     OS << Stack.front().String;
 
   return true;
 }

llvm/unittests/DebugInfo/DWARF/DWARFExpressionCompactPrinterTest.cpp

TestExprPrinter({DW_OP_entry_value, 0x01, DW_OP_reg0, DW_OP_stack_value},
                   "entry(R0)");
 }
 
 TEST_F(DWARFExpressionCompactPrinterTest, Test_OP_entry_value_mem) {
   TestExprPrinter(
       {DW_OP_entry_value, 0x02, DW_OP_breg13, 0x10, DW_OP_stack_value},
       "entry([SP+16])");
 }
+
+TEST_F(DWARFExpressionCompactPrinterTest, Test_OP_nop) {
+  TestExprPrinter({DW_OP_nop}, "<stack of size 0, expected 1>");
+}
+
+TEST_F(DWARFExpressionCompactPrinterTest, Test_OP_nop_OP_reg) {
+  TestExprPrinter({DW_OP_nop, DW_OP_reg0}, "R0");
+}